Monday, April 15, 2013

Electrical Wiring Diagram Star Delta Control and Power Circuit Using Mitsubishi PLC Program

This article is intended to diagrammatically demonstrate the concept of the wiring principle of a star delta (wye delta) motor control and power circuit when using a PLC (Programmable Logic Controller) ladder program to control the switching of a 440 volts induction motor.

The electrical components comprising the hardware parts of the circuit includes:

1.) 440 volts to 220 volts step-down transformer - 1 piece
2.) 220 volts to 24 volts step-down transformer - 1 piece
3.) 24VAC to 24VDC power supply converter - 1 piece
4.) Normally-open push button switch - 2 pieces
5.) Mitsubishi PLC (Programmable Logic Controller) Type FX2N-16MR-ES - 1 unit
6.) Auxiliary relay with 24VDC coil and minimum 1 Normally-open contact - 3 pieces
7.) 3 phase 440 volts AC NFB (No-fuse Breaker) circuit breaker (ampere rating depends on motor capacity) - 1 piece
8.) 3 phase 440 volts AC magnectic contactor with 220 VAC coil (ampere rating depends on motor capacity) - 3 pieces
9.) Thermal overload relay (ampere rating depends on motor capacity) - 1 piece
10) 440 VAC induction motor (motor capacity depends on application) - 1 unit

Fig. 1: Electrical Wiring Diagram of Star Delta Power and Control Circuit with PLC Connection:

(Open in new window for a more legible view)

Circuit Description:
The power circuit of the induction motor is physically wired in the usual configuration for star delta connection. There are 3 magnetic contactors used in the power circuit, the main magnetic contactor is connected directly to the primary coil terminal of the motor U1-V1-W1 while the star and delta magnetic contactors are connected to the secondary coil terminal of the motor U2-V2-W2.

The control circuit for the switching sequence of the motor is not physically wired but is rather constructed in the internal program of the PLC with only the start and stop push button switches and the thermal overload relay contact externally connected to the PLC input terminals with X0 for the Start Push Button Switch, X1 for the Stop Push Button Switch and X2 for the Thermal Overload Contact illustrated in Figure 2 below, along with the three auxiliary relays corresponding to each of the three magnetic contactors externally connected to the output terminals of the PLC with Y0 for the Main Contactor Coil Relay, Y1 for the Star Contactor Coil Relay and Y2 for the Delta Contactor Coil Relay shown in Figure 3 below.

Star Delta Mitsubishi PLC External Input Terminal Connection
Fig. 2: Star Delta Mitsubishi PLC External Input Terminal Connection

Star Delta Mitsubishi PLC External Output Terminal Connection
Fig. 3: Star Delta Mitsubishi PLC External Output Terminal Connection
With reference to Figure 1 above, the power supply for the circuit consists of a 440 to 220 volts step-down transformer to supply AC voltage to power up the PLC and also for the energizing coil of the maagnetic contactors. Another 220 volts to 24 volts AC step-down transformer is included and converted to DC voltage by the 24 volts DC converter to provide 24 Volts DC supply voltage to the energizing coil of the auxiliary relays. The isolated normally-open contact points of these auxiliary relays serves as switching contacts to sequentially energize the coils of each of the three magnetic contactors.

All the sequential switching function of the control circuit is manipulated by the internal program constructed inside the PLC memory, which is easily done by drawing the wiring diagram of the logic sequence using the ladder programming editor of the Mitsubishi PLC software as shown below.

Note the assigned bit numbers in the PLC ladder program corresponds to the physical designated address points of the external components wired to the PLC digital input and output terminals. X000 for X0 start push button switch, X001 for X1 stop push button switch and X002 for X2 thermal overload contact are all correlated to the actual connection of the input components in the PLC digital input terminals, while the Y000 for Y0 main contactor relay, Y001 for Y1 star contactor relay and Y002 for Y2 delta contactor relay are the assigned bit address also matching the location of the connected output components in the PLC digital output terminals.
Mitsubishi PLC Ladder Logic Program for the Star Delta motor control circuit
Fig. 4: Mitsubishi PLC Ladder Logic Program for the Star Delta motor control circuit
The operational sequence of the circuit constructed in the ladder logic program of the PLC software works in the same way as it would normally behave in a hard-wired star delta control circuit using physical electrical components such as relays and timers.

When the start push button is switch on in the external input terminal of the PLC, its corresponding input terminal address X0 activates the assigned software bit address X000 in the program which activates bit address Y000 to energize the main contactor relay externally connected to the PLC output terminal Y0.

Y000 is connected in parallel to X000 to act as a maintaining contact to hold the circuit after releasing the start push button switch X000, which also activates Y001 to energize the star contactor relay as well as timer T0. Once the specified time period of T0 is reached, the program transfers command to release the star contactor in Y001 then energizes the delta contactor in Y002.

Safety interlocks are also provided in the program which are alternately connected between the output bits Y001 and Y002 so that one will not activate without the other deactivated first, in which case, Y001 will not activate unless Y002 is deactivated, whereas Y002 will not also be allowed to activate without firstly deactivating Y001.

Two series contacts are provided to stop running the program. The stop push button switch of bit address X001 and the thermal overload switch of bit address X002 which both serves as series connected disconnecting means to deactivate output bit Y000 which would release its corresponding holding contact connected in parallel across start push button switch bit X000.

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